Carol Foss, director of conservation for the NH Audubon Society, will lead a discussion on the subject Friday at 7 p.m. at the Massabesic Audubon Center.

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Susan Arnold, of the Appalachian Mountain Club, said while officials for the Portland-Montreal Pipe Line continue to say there is no current proposal on the table to move tar sands from Western Canada east to Portland, there is concern a Keystone XL of the East could be proposed for the line in the future.

Sean Mahoney, vice president of the New England-based Conservation Law Foundation, said the Canadian pipeline company Enbridge and PMPL might say they have no plan the idea for a reversal of the direction of line which takes oil from tanker ships in South Portland's terminal and transports it through the northern tip of New Hampshire west into Canada, there could be change in the future to have tar sands move this way.

The permit has been extended for the line to be re-purposed.

"Yes, Canada is definitely trying to get the tar sands to market," he said in a 2012 interview. "They have a glut in the Midwest and it affects their prices. The fact they filed these applications makes it clear the intent is there and from what we can tell of the action in Canada from Enbridge, they absolutely want to connect the dots from western Canada to Quebec to Maine. The pipeline corporation and Enbridge say they don't, but their actions speak more than their denial."

He and other environmentalists argue the universal cost of extracting the bitumen-clay-sand product and the risk of a spill are too high for such a valuable tract of land.

But others say it would be foolish to stand in the way of the production of petroleum products from a friendly neighbor like Canada.

Mahoney said, "it is an old pipeline and it goes through some very sensitive areas," and a spill could devastate the Connecticut and Androscoggin Rivers and drinking water supply for most of southern Maine for "comparatively little economic benefit."

Since 1941, the pipeline -- actually two pipes 18 and 24 inches in diameter -- have been moving foreign oil West from Portland harbor to refineries in Montreal. It was established during World War II as a safe way to ship light crude to Canada.

About 35 miles of the 236-mile line is located across the northern tip of the state entering at Shelburne north of Gorham and moving through the Stark-Groveton area before exiting the state at Guildhall, Vt. It is different than the Portland natural gas pipe that was primarily co-located along the line in the 1990s.

PMPL officials noted the line has an outstanding safety record and received numerous awards.

Currently about 170,000 barrels of oil move East to West on the pipeline daily but the line has twice that capacity, officials said.

Tar sands is a combination of clay sand water and bitumen. It differs from conventional oil in terms of extraction and in the processes used to make it into a product.

Where oil is drilled from the ground, tar sands are mined or stripped from the ground, requiring the removal of trees and topsoil to get at the gritty tar-smelling substance.

It needs to be heated to extract the product and that requires energy.

Western Canada, particularly Alberta, is rich in tar sands oil and its export including the Keystone XL Pipe line proposed by TransAmerica would go south to north from Canada to Texas. But development of a East-West corridor for the product is still developing.

Last year, federal regulators in Canada gave approval to the pipeline giant Enbridge to reverse direction of one of its westward pipelines that would allow tar sands to flow east into Montreal.

The company has publicly said that the line ends in Montreal and would not continue East into the United States.

Jim Murphy, senior counsel for the National Wildlife Federation, based in Montpelier, Vt. said he believes in the future that line could continue East, allowing tar sands to make its way to Portland where it could be moved by sea to refineries in St. John, New Bruswick, Canada and other locations to make it useable for gasoline and other petroleum-based products.

He said the process of bringing tar sands oil to market is a "very destructive and energy-dependent process," which he said he believes moves the country's fuel supply in the wrong direction.

Murphy said he thinks, "The chances are pretty high, based on the general economic factors that, there will be a lot of pressure to do this," here in New England in the coming years.

Kenneth D. Kimball, Appalachian Mountain Club's director of research, said the idea could have implications for a very sensitive stretch of the White Mountains.

Murphy argues that moving this particular product has not been adequately studied, particularly on an old line like this, which he said was set up to handle a much less corrosive product.

In a spill, such as one in Kalamazoo, Mich., in which at least 840,000 gallons broke free from an Enbridge pipeline, toxic gases were released into the air and the cleanup was difficult, Murphy said.

Such a spill "would devastate the Connecticut River or the Androscoggin," Murphy said and Sebago Lake, which provides much of the drinking water for southern Maine.